1. Field of the Invention
This invention relates generally to photoelectrochemical energy conversion techniques and, more specifically, to the application of superlattice semiconductors as photoelectrodes in photoelectrochemical energy conversion processes.
2. Description of the Prior Art
Photovoltaic cells, commonly known as solar cells, are essentially semiconductors that have capability of converting electromagnetic energy, such as light or solar radiation, directly to electricity. Such semiconductors are usually characterized by solid crystalline structures that have energy band gaps between their valence electron bands and their conduction electron bands. Free electrons normally cannot exist or remain in these band gaps. However, such photovoltaic cells are also characterized by materials of a type that when light is absorbed by them, electrons that occupy low-energy states are excited to jump the band gap to unoccupied higher-energy states. For example, when electrons in the valence band of a semiconductor absorb sufficient energy from photons of the solar radiation, they can jump the band gap to the higher-energy conduction band.
Electrons so excited to higher-energy states leave behind them unoccupied low-energy positions or "holes." Such "holes" can shift from atom to atom in the crystal lattice; thus, the holes act as charge carriers, as do free electrons, and contribute to the crystal's conductivity. Therefore, most of the photons that are absorbed in the semiconductor give rise to such electron-hole pairs. It is these electron-hole pairs that generate the photocurrent and, in turn, the photovoltage exhibited by solar cells.
These electron-hole pairs produced by the light would eventually recombine, thereby converting to heat or a photon the energy initially used to jump the band gap, unless prohibited from doing so. Therefore, a local electric field is created in the semiconductor by "doping" or interfacing dissimilar materials to produce a space-charge layer. This space-charge layer serves to separate the holes and electrons for use as charge carriers. Once separated, these collected hole and electron charge carriers produce a space charge that results in a voltage across the junction, which is the photovoltage. If these separated hole and charge carriers are allowed to flow through an external load before recombining, they constitute a photocurrent.
The photocurrent generated in a solar cell can be utilized in a number of ways. It can be collected at solid contacts or electrodes on the semiconductor and directed by conductors through external electrical loads to perform useful work. Also, much work has been done in the field of utilizing the photocurrent internally in an electrolysis processes. That is, the separated electron-hole pairs are utilized immediately in a photoelectrochemical cell to drive chemical oxidation and reduction reactions on the surfaces of the semiconductor electrode and counter electrode in the cell. See, for example, Robert T. Ross and Arthur J. Nozik, "Efficiency of Hot-Carrier Solar Energy Converters", Journal of Applied Physics, vol. 53, pp. 3813-3818 (1982); A. J. Nozik, "Photoelectrochemical Devices for Solar Energy Conversion", Photovoltaic and Photoelectrochemical Solar Energy Conversion, pp. 263-312 (Plenum Publishing Corporation, 1981); Arthur J. Nozik, "Introductory Lecture: Photoelectrochemistry", Faraday Discussions of the Royal Society of Chemistry, No. 70, Photoelectrochemistry (1980); A. J. Nozik, et al. "Charge Transfer at Illuminated Semiconductor-Electrolyte Interfaces", Interfacial Photoprocesses: Energy Conversion and Synthesis, Advances in Chemistry Series vol. 184, American Chemical Society (1980), based on a symposium sponsored by the Division of Colloid and Surface Chemistry at the 176th meeting of the American Chemical Society, Miami Beach, Fla., Sept. 11-13, 1978; Arthur J. Nozik, "Photoelectrochemistry: Applications to Solar Energy Conversion," Ann. Rev. Phys. Chem, vol. 29, pp. 189-222 (1978); Gerald Cooper, et al. "Hot Carrier Injection of Photogenerated Electrons at Indium Phosphide-Electrolyte Interfaces", Journal of Applied Physics, vol. 54, pp. 6463-6473 (1983); John A. Turner and Arthur J. Nozik, "Evidence for Hot-Electron Injection Across p-GaP/Electrolyte Junctions", Applied Physics Letters, vol. 41, pp. 101-103 (1982); D. S. Boudreaux, et al. "Hot Carrier Injection at Semiconductor-Electrolyte Junctions", Journal of Applied Physics, vol. 51, pp. 2158-2163 (1980); F. Williams and A. J. Nozik, "Solid State Perspectives of the Photoelectrochemistry of Semiconductor-Electrolyte Junctions", Nature, vol. 311, pp. 21-27 (1984); J. A. Turner, et al. "Photoelectrochemistry with p-S Electrodes: Effects of Inversion", Applied Physics Letters, vol. 37, pp. 488-491 (1980); J. A. Turner, "SupraBand-Edge Reactions at Semiconductor-Electrolyte Interfaces: Band-Edge Unpinning Produced by the Effects of Inversion", American Chemical Society Symposium Series (1980).
Electrolysis is, of course, the decomposition of a chemical compound by an electrode current. For example, it is a common process to decompose an electrolyte comprising water, i.e., H.sub.2 O into its constituent elements of hydrogen and oxygen in a redox reaction generally described as: EQU 2H.sub.2 O.fwdarw.2H.sub.2 +O.sub.2
where EQU 4h.sup.+ +2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+
and EQU 2(2H.sup.+)+2(2e.sup.-).fwdarw.2H.sub.2
In such an electrolysis process, the hydrogen gas (H.sub.2) bubbles off the negative electrode or cathode, and the oxygen gas (O.sub.2) bubbles off the positive electrode or anode. The hydrogen and oxygen gases can, of course, be put to many wellknown beneficial uses, including the production of fuel.
One of the historic impediments to the large-scale use of electrolysis processes for the production of fuel has been the inefficiencies of such processes. Specifically, the electric energy input required to drive the redox reaction is not justified by the energy output available from the fuel derived in the process. However, interest in this area has been increased by the possible use of solar radiation to drive the redox reaction through the use of solar cell semiconductors as electrodes in the electrolysis process.
Such processes, now commonly referred to as photoelectrochemical energy conversion, are the subject of the previously-cited references. They essentially comprise immersing a semiconductor material in a liquid electrolyte and exposing the semiconductor material to light. The semiconductor-liquid interface creates the local electric field to produce the depletion zone (or space charge) and under illumination generates a voltage and current across the semiconductor-liquid interface. Such an interface is essentially like a Schottky semiconductor heterojunction formed by an interface of different materials, although the term Schottky heterojunction is usually used to refer to a solid semiconductor-metal interface.
When exposed to light, the internal photocurrent induced in the semiconductor drives the electrolysis reaction. Essentially, the photoexcited charge carriers (electrons that have jumped the band gap to the higher energy level conduction band) are injected into the electrolyte from the semiconductor before the electron-hole pairs can recombine across the band gap. Such charge carriers injected into the electrolyte take part in the redox chemical reaction in the electrolysis process.
It is known, as also discussed in the previously-cited references, that photon energies in excess of the threshold energy gap or band gap between the valence and conduction bands are usually dissipated as heat, and thus are wasted and do no useful work. More specifically, there is a fixed guantum of potential energy difference across the band gap in the semiconductor. In order for an electron in the lower-energy valence band to be excited to jump the band gap to the higher-energy conduction band, it has to absorb a sufficient quantum of energy, usually from an absorbed photon, with a value at least equal to the potential energy difference across the band gap.
If the electron absorbs less than that required for the threshold quantum of energy, it will not be able to make the jump across the band gap. Such energy is essentially lost for practical purposes.
On the other hand, if the electron absorbs more than the threshold quantum of energy, e.g., from a larger energy photon, it can jump the band gap. The excess of such absorbed energy over the threshold quantum required for the electron to jump the band gap results in the electron being higher in energy than most of the other electrons in the conduction band. Such electrons having energy levels higher than the lower edge of the conduction band, i.e., the top edge of the band gap, are referred to as "hot electrons". For every electron excited out of its normal energy level, there is a corresponding "hole". Thus, for each hot electron, there can be a corresponding hot hole, both of which are generally referred to as "hot carriers".
Hot carriers usually lose their excess energy to the host lattice very rapidly in the form of heat. This process, in which the hot carriers dissipate their excess energy to the host lattice and equilibrate with the lattice at ambient temperature, is known as thermalization. Such thermalization of hot carriers results in the carriers being reduced in energy to the energy level at the edge of the conduction band. Since such thermalization normally occurs in about 10.sup.-12 seconds, the photocurrent delivered to a load or injected into an electrolyte comprises carriers having energy levels at the lower edge of the conduction band. In other words, the effective photovoltage of a single band gap semiconductor is limited by the band gap.
The practical effect of this limitation prior to this invention was that the semiconductor designer had to sacrifice efficiencies in one area in order to achieve them in another. Specifically, in order to capture as many photons from the spectrum of solar radiation as possible, the semiconductor had to be designed with a small band gap so that even small photons from lower energy radiation could excite electrons to jump the band gap. However, in doing so, there were at least two negative effects that had to be traded. First, the small band gap resulted in a low photovoltage device, thus low power output. Second, the more energetic photons from higher energy radiation produced many hot carriers having much excess energy that would be lost as heat upon almost immediate thermalization of these hot carriers to the edge of the conduction band. On the other hand, if the semiconductor is designed with a larger band gap to increase the photovoltage and reduce energy loss caused by thermalization of hot carriers, then the smaller photons from lower-energy radiation will not be absorbed.
Consequently, prior to this invention, it was necessary to balance these considerations and try to design a semiconductor with an optimum band gap, realizing that in the balance, there had to be a significant loss of energy from both large and small energy photons. It has been calculated that the theoretical maximum energy conversion with conventional single band gap semiconductors is about 31%. However, if all the photon energy from the visible light spectrum could be captured and used, the theoretical conversion efficiency of a semiconductor would be about 68%.
Many of the previously-cited references are directed at attempts to increase conversion efficiency by capturing and utilizing the excess energy of hot carriers by injecting them into an electrolyte for driving redox reactions before they thermalize. The theory of such attempts is that if the thermalization time of hot carriers was greater than their residence time in the semiconductor, then hot-carrier injection into the electrolyte could occur. However, because of the extremely rapid thermalization of the hot carriers, which occurs in about a picosecond in bulk semiconductors, no one has been able to achieve this goal prior to this invention.
It had been throught prior to this invention that it was necessary to either slow down the thermalization rate significantly or to find a way to remove the hot carriers in less than a picosecond. For example, in the previously-cited reference, Robert T. Ross and Arthur J. Nozik, "Efficiency of Hot-Carrier Solar Energy Converters", Journal of Applied Physics, vol. 53, pp. 3813-3818 (1982), it was suggested that with highly doped semiconductors used in combination with semiconductor-liquid or semiconductor-solid interfaces where large electric fields exist because of initial chemical potential differences between the phases, resulting quantization effects in the space charge layer would slow down the thermalization process and enhance hot carrier charge transfer out of the semiconductor. However, nothing close to the ultimate theoretical conversion efficiencies of 66% could be attaind by such systems because most of the thermalization of photogenerated hot carriers occurs in the much larger bulk region of the semiconductor (where the energy bands are flat) rather than in the small depletion zone or space-charged region where the band bending and quantization effects occur.
Consequently, there remains a need for a device that can capture and utilize a large range of photon energies from the solar radiation spectrum, yet not sacrifice either photovoltage or excess energy loss to heat by thermalization of hot carriers. This need has remained unsatisfied until this invention.
Another development that was occuring about the same period as the attempts just described to utilize photoelectrochemical processes for solar energy conversion, although in different research endeavors, was the development of superlattice semiconductor structures. Much of the development, experimentation, observations, and theories of such superlattice semiconductor structures are reported or described in the following prior art references: R. Dingle, et al. "Direct Observation of Superlattice Formation in a Semiconductor Heterostructure", Physics Review Letters, vol. 34, pp. 1327-1330 (1975); Raymond Dingle, "Confined Carrier Quantum States in Ultrathin Semiconductor Heterostructures", Feskorterproblem, vol. 15, pp. 21-48 (1975); P. L. Gourley and R. M. Biefield, "Growth and Photoluminescence Characterization of a GaAx.sub.x P.sub.1-x /GaP Strained-Layer Superlattice", Journal of Vacuum Science Technology, vol. 21, pp. 473-475 (1982); John A. Moriaty and Srinivasan Krishnamurthy, "Theory of Silicon Superlattices: Electronic Structure and Enhanced Mobility", Journal of Applied Physics, vol. 54, pp. 1892-1902 (1983); L. L. Chang, "A Review of Recent Advances in Semiconductor Superlattices", Journal of Vacuum Science Technology B, vol. 1, pp. 120-125 (1983); Gottfried H. Dohler, "Solid State Superlattices", Scientific American, vol. 249, pp. 144-151 (1984); P. L. Gourley and R. M. Biefield, "Quantum Size Effects in GaAs/GaAs.sub.x P.sub.1-x Strained-Layer Superlattices," Applied Physics Letters, vol. 45, pp. 749-751 (1984); and Venkatesh Narayanamurti, "Crystalline Semiconductor Heterostructures", Physics Today, pp. 24-32 (October 1984).
Essentially, superlattice structures are specially structured semiconductors in which two materials with different electronic properties are interleaved in thin layers by depositing sheets of two semiconducting materials in alternation or by introducing impurities into alternating layers of a single semiconducting material. Superlattices comprising alternating ultrathin layers of two different semiconductors are known as compositional superlattices. Such compositional superlattices, in which the alternating materials have crystallized structures that do not closely match each other in interatomic lattice distances at their interface, are called strained-layer superlattices. On the other hand, a periodic array consisting of layers of the same semiconductor doped in two different ways is known as a doping superlattice.
Each alternating layer of semiconductor material or doping has a different band gap than its adjacent layers. This periodic alternation of layers, therefore, gives rise to a periodic alternation in electric potential. Each layer of the semiconductor with the smaller band gap produces what is called a potential well. Thus, the term "multiple-quantum-well" or "MQW" is often used to describe such semiconductors, particularly when the adjacent layers with the larger band gaps are thick enough to provide an effective barrier to the transfer of charge carriers in the conduction bands of the well layer.
It is known that the potential or quantum wells split the valence bands and the conduction bands into a plurality of minibands. These minibands are narrower than the bands in a bulk semiconductor and are separated from each other in the conduction band by relatively large minigaps. A result of this structure is that electron-hole pairs can be held separated for longer periods before they recombine. Also, photons are absorbed only if their energies equal or exceed a threshold value determined not by the band gap of either semiconductor alone, but by the effective band gap, which is the difference in energy between the least energetic conduction miniband and the most energetic valence miniband. Further, photon absorption is maximized where photon energies correspond to the differences in energy between pairs of minibands.
Another significant characteristic of superlattice semiconductors is that electronic properties of the superlattices can be designed. Specifically, values of energy levels available to electrons can be tailored by the appropriate choices of semiconductor materials or doping. Also, the widths of the minibands can be tailored.
While superlattice semiconductors exhibit many interesting characteristics and potential capabilities, their use in practical energy conversion applications has been somewhat limited by physical constraints in fabrication techniques, particularly relating to electrical connections or contacts. Specifically, prior to this invention, there was no known method or structure of drawing off the charge carriers from a superlattice semiconductor at the respective discrete and higher-energy levels of those charge carriers in the superlattice. Therefore, the use of superlattices has been primarily experimental for learning about their properties and potential capabilities. Some advances have been made in using superlattices in electronic control applications, such as transistors, oscillators, modulators, and the like. However, prior to this invention, there have been no practical uses made of superlattices as the photoactive element in solar energy conversion or particularly in the field of photoelectrochemistry. The present invention brings together for the first time knowledge from the two disciplines of photoelectrochemistry and solid-state superlattice semiconductors to provide breakthroughs both in efficient photoelectrochemical solar energy conversion and in a method of utilizing the beneficial capabilities of superlattice and multiple-quantum-well (MQW) semiconductors for solar energy conversion.